In recent years, the following sheet has been developed and has been used for various applications such as vessels: an IF (interstitial free) steel sheet, in which the content of C is reduced to tens of parts per million by vacuum degassing and which is made free from solutes C and N by adding a trace amount of a carbonitride-forming element such as Ti, Nb, or the like. The IF steel sheet, which is free from solutes C and N, does not have age-hardenabilty and has excellent workability. Therefore, in many cases, the IF steel sheet is used as a steel sheet for vessels, which is required to have high formability including drawing. However, reducing the content of C in molten steel increases the amount of dissolved oxygen as described in Patent Literature 1 and therefore there is a problem that the amount of inclusions such as alumina is increased.
From the viewpoint of global environmental conservation and the like, demands for reducing the amount of steel used by the gauge reduction of steel sheets are recently growing. If the gauge of the IF steel sheet is reduced in accordance with such demands, then inclusions are likely to appear on the surface thereof and a problem that defects are likely to extend through the steel sheet occurs in the case of an extremely thin material. On the other hand, in low-carbon steel sheets (since the content of C is not extremely reduced, the amount of inclusions is small and the problem that inclusions are likely to appear on the surface does not occur), age hardening occurs to reduce the formability thereof and therefore problems such as press cracking are likely to occur during gauge reduction.
Therefore, a low-carbon steel sheet which contains a few inclusions and which does not have age-hardenabilty is strongly demanded in association with the gauge reduction of such steel sheets.
For such a demand, for example, Patent Literature 1 discloses a high-strength steel sheet for forming. The high-strength steel sheet contains, in percent by mass, C: 0.01% to less than 0.1%, Si: 0.1% to 1.2%, Mn: 3.0% or less, Ti: the ratio (effective *Ti)/C being 4 to 12, B: 0.0005% to 0.005%, Al: 0.1% or less, P: 0.1% or less, S: 0.02% or less, and N: 0.005% or less, where effective *Ti is defined by the equation effective *Ti=Ti−1.5S−3.43N. According to a technique disclosed in Patent Literature 1, even in a low-C steel sheet in which the content of C is increased, by allowing a large amount of Si to be contained thereby promoting elimination of C from ferrite, and by adjusting the ratio effective *Ti/C to 4 to 12, solutes C, N, S, and the like can be completely fixed, the in-plane anisotropy is small, the yield ratio is low, aging is completely suppressed, and softening by high-temperature heating can be prevented.
Patent Literature 2 discloses a steel sheet which contains, in percent by mass, C: 0.0080% to 0.0200%, Si: 0.02% or less, Mn: 0.15% to 0.25%, Al: 0.065% to 0.200%, N: 0.0035% or less, and Ti: 0.5≦(Ti−(48/14)N−(48/32)S)/(48/12)C)≦2.0. The steel sheet has an average grain diameter of 20.0 μm or less and low anisotropy. According to a technique disclosed in Patent Literature 2, the following sheet is obtained: a steel sheet in which the cold-rolling ratio dependence of Δr which is an indicator for in-plane anisotropy is low and the change in Δr due to variations in production conditions is small.    [PTL 1] Japanese Unexamined Patent Application Publication No. 5-5156    [PTL 2] Japanese Unexamined Patent Application Publication No. 2007-9272    NPL 1: The Japan Institute of Metals, Kinzoku Kagaku Nyumon Shirizu 2 Tekko Seiren, p. 195, July 2000